This disclosure relates generally to refrigerant vapor compression systems and, more particularly, to detecting and defrosting the heat exchanger coil of an evaporator of a refrigerant vapor compression system when supplying cold air to a temperature controlled space being maintained at a temperature below the freezing point of water (32° F./0° C.).
Refrigerant vapor compression systems used in connection with transport refrigeration systems are generally subject to stringent operating conditions due to the wide range of operating load conditions and the wide range of outdoor ambient conditions over which the refrigerant vapor compression system must operate to maintain product within the cargo space at a desired temperature. The desired temperature at which the cargo needs to be controlled can also vary over a wide range depending on the nature of cargo to be preserved. For example, for fresh products, such as produce, dairy products, fresh meats, fresh poultry, the control set point air temperature returning from the controlled temperature space to the evaporator may typically range from 34° F. up to 86° F. (1° C. to 30° C.), while for frozen products, such as ice cream, seafood, frozen meat and poultry, and other frozen items, the control set point air temperature typically may range from 32° F. down to −30° F. (0° C. to −34.4° C.).
When the refrigerant vapor compression system is operating in a frozen temperature control mode for maintaining air temperature within a temperature controlled space below 32° F. (0° C.), the temperature of the refrigerant will be so low that the heat transfer surfaces of the evaporator coil will be less than 32° F. (0° C.). Thus moisture in the air returning to the evaporator from the temperature controlled space will deposit as ice on the heat transfer surfaces of the evaporator coil. As ice builds up on the evaporator coil, the air flow rate is reduced because the build-up of ice blocks off portions of the air flow passages over the evaporator coil.
Additionally, the build-up of ice on the exposed heat transfer surfaces of the evaporator coil creates additional thermal resistance to the transfer of heat from the air flow to the refrigerant passing through the heat exchange tubes of the evaporator coil, thereby degrading the heat transfer performance of the evaporator coil and lowering the cooling capacity of the evaporator coil. As the evaporator coil cooling capacity decreases, the lesser amount of refrigerant that can be evaporated in passing through the evaporator coil. In response to the reduced cooling capacity, the evaporator expansion valve reduces its flow opening to reduce the mass flow of refrigerant passing through the evaporator coil. As a consequence, the refrigerant pressure within the evaporator coil and downstream thereof, including the refrigerant at the suction inlet to the compressor, referred to as the suction pressure, is lowered. If the suction pressure drops below a preset lower limit, the system will cycle off to avoid possible damage to the compressor. However, as a cooling demand is still imposed on the system, the system will cycle back on. An undesirable on-off cycling of the compressor may ensue.